Provide more context for rustc +nightly -Zunstable-options on stable

Author: jieyouxu

src/pretty_clif.rs

@@ -225,10 +225,10 @@ pub(crate) fn write_ir_file(
     let res = std::fs::File::create(clif_file_name).and_then(|mut file| write(&mut file));
     if let Err(err) = res {
         // Using early_warn as no Session is available here
-        rustc_session::early_warn(
+        let handler = rustc_session::EarlyErrorHandler::new(
             rustc_session::config::ErrorOutputType::default(),
-            format!("error writing ir file: {}", err),
         );
+        handler.early_warn(format!("error writing ir file: {}", err));
     }
 }